PI (polyimide) film is a layer of even alignment film on a surface of TFT (Thin Film Transistor) substrate and CF (Color Filter) substrate, it makes liquid crystal molecules to be arranged according to a certain pre-tilt angle in order to control a passing direction of light, and achieve a display purpose by applying an appropriate driving voltage. Currently, there are mainly two coating methods of coating technique for PI film: PI roller coating and PI inkjet.
Referring to FIG. 1, a process for transfer-printing technique mainly includes: a polyimide (PI) solution is ejected from a PI nozzle 4 and it drips on a turning metal roller 3, a scraper 5 is used to scrape excessive PI solution, the metal roller 3 turns and is in contact with a plate cylinder 6, and the PI solution is stored on a printing plate 7 which is attached on the plate cylinder 6, then the plate cylinder 6 turns and is in contact with a glass base plate 1 on a printing stage 2, finally the PI solution is transfer-printed on the glass base plate 1. The roller coating technique has the following drawbacks: cumbersome equipment, poor mobility, large consumption of consumables (dummy base plates and printing plates), thickness film which is difficult to adjust, complicated operation, low PI usage percentage, high operating costs and common deficiencies easily caused by polluted printing plates.
Referring to FIGS. 2, 3, 4a and 4b, PI inkjet is controlled by using deformation of piezoelectric ceramics, so that a PI drip 8 is dripped from a nozzle hole 9 and land on a glass base plate 1 disposed on a printing stage 2, the PI drip 8 is then evenly spread on the glass base plate 1, and an even film is formed by diffusion of the PI drip 8. This technical method has the following drawbacks: large uneven area of edges of the film thickness, small nozzle hole which is easy to be clogged, interval difference of the nozzle hole 9 and uneven spreading which will produce deficient products with uneven inner lines and inner surfaces. The most serious deficiency is that, the PI drip 8 is unable to be spread evenly to form a film when a segmental difference of the base plate surface is too large, as a result, product quality is substantially decreased and even large quantity of products is rejected. Furthermore, for the printing method of PI inkjet, because of the spreading of the drips, thus a halo area shown in FIG. 4a and an edge wave shown in FIG. 4b can be easily formed, which will affect the quality of PI film.
US patent number 20060172091 discloses a forming method of alignment film, by forming a layer of hydrophobic film on parts of a surface of a TFT substrate corresponding to a non-display area, and forming a layer of hydrophilic film on parts corresponding to a display area, so that a boundary of the hydrophilic film is limited by the hydrophobic film, and by employing the inkjet printing method, an alignment film is then formed by dripping of a liquid of the material of the alignment film. Such forming method of employing inkjet printing on the areas corresponding to the hydrophilic film is the same as the conventional technique illustrated in FIG. 3. Even though the deficiency of substantially decreased product quality and large quantity of rejected products caused by the drips being unable to be spread evenly to form a film when a segmental difference of the base plate surface is too large, can be avoided to a certain degree; the problems of halo area shown in FIG. 4a and edge wave shown in FIG. 4b still exist, which will affect the quality of the alignment film. Therefore, the inkjet method of alignment film of conventional coating technique needs to be improved.